The present invention relates to an ADF (Automatic Document Feeder) mounted on an image forming apparatus and more particularly to an ADF of the type sending out a document via an image reading position while warping it.
An ADF of the type described is conventional with a copier, facsimile apparatus or similar image forming apparatus. Japanese Patent Laid-Open Publication No. 10-72141, for example, discloses ADF applicable to an image forming apparatus of the type reading a document by receiving a reflection from the document while holding optics in a halt below a slit glass shorter than the document. The ADF taught in this document conveys a document via the slit glass in a so-called sheet-through fashion.
Another conventional ADF is applicable to an image forming apparatus of the type stopping a document on a contact glass having a greater area than the document, and then moving optics to scan the document. Such an ADF sends a document to the contact glass, stops the document at the contact glass, and then sends it out after the document has been read.
In any case, should a document be conveyed in a plane that is the extension of the image reading position, the overall area of the ADF would increase. To make the ADF compact, it has been customary to send in a document to the image reading position while warping and reversing it from above the reading position, send it out from the image reading position while also warping it, and again reversing, if necessary, the document before the document is driven out to a tray.
The prerequisite with the sheet-through type ADF is that it conveys a document being read at a constant speed because the document conveying speed has critical influence on reading accuracy. In practice, however, the leading edge of a document moved away from the glass platen advances along a guide surface, abuts against one of a pair of rollers located downstream of the guide surface, and then enters a nip between the rollers. Generally, each roller has a smooth surface implemented by resin so as not to contaminate a document. This, coupled with resistance ascribable to the curvature of the roller, causes the document to slip on the roller before entering the above nip. The slip effects the document conveying speed and causes the document to temporarily stay at the reading position, causing the resulting image to be elongated.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Publication No. 7-120094 and Japanese Utility Model Publication No. 2,515,306.